We wove through the morning traffic of Tacoma and Olympia (luckily most everyone was headed the opposite direction). We reached the village of Blyn near Port Angeles by 9 AM or so where we stopped at a roadside rest to have our first introductory presentations on the geology and anthropology of the Puget Sound and Olympic Peninsula. The skies were somewhat overcast, which was worrisome because a storm was rolling in soon, and we had hopes of having a clear view of the Olympic Mountains from Hurricane Ridge.
To my great relief, we broke through the clouds and drove into bright sunlight. We climbed the winding highway through the thick forest with increasingly far-ranging views. But nothing quite prepares anyone for the view from the end of the paved road at Hurricane Ridge. It is simply astounding.
Subduction is the story of the Pacific Northwest. For most of 200 million years a convergent boundary has been active in the region, as the crust of the Pacific Ocean basin has been sinking against the edge of the North American Continent. In some places, for instance California, the subduction zone has been replaced by a transform boundary (the San Andreas fault). But in Northern California, Oregon, Washington, and part of British Columbia, the subduction zone is still active, still producing earthquakes, and still raising mountains. It's called the Cascadia Subduction Zone
|Source: Geological Society of America|
In a "normal" subduction zone, there are four parts: the trench, an accretionary wedge, a forearc basin, and a magmatic arc. The trench is the deepest part of the ocean floor where the oceanic crust sinks back into the mantle. The accretionary wedge is a collection of seafloor sediments and crust that has been scraped off the subducting plate and added to the edge of the continent. The forearc basin is a shallow sea that may develop inland of the accretionary wedge (California's Great Valley originated in this fashion). The magmatic arc is a system of volcanoes and intrusive plutons resulting from the melting of rocks in the lower crust and upper mantle above the descending slab (water released from the slab lowers the melting point of the rock, leading to the formation of the molten rock). Other features may develop, depending on the angle of subduction or the geometry of the plate boundary. There will be more on those later in the series.
The geologic map reveals the basic structure of the Olympics. A "horseshoe" of basalt and sedimentary rocks (the Peripheral Rocks, or Crescent Formation) partially surrounds the "Core Rocks", an assemblage of lightly metamorphosed sandstone and shale layers. The Core Rocks are characteristic of the types of deposits that form from underwater landslides ("turbidity currents") within the trench and accretionary wedge of a subduction zone. The fact that these rocks are now thousands of feet above sea level is the interesting conundrum. Accretionary wedges are generally below sea level, or exist as small islands. They can be pushed higher. For instance, the rocks of the Cascadia accretionary wedge are exposed in the Coast Ranges of Washington, Oregon and California, but nowhere are the exposures as spectacular as the Olympic Mountains.
Sorry, this trench is full..."). The mountains have been rising for around 15 million years. They would be higher, but the incredible amount of precipitation tears the mountains down at a roughly equivalent rate.
When we first visited Hurricane Ridge the prior week, the highest peaks were obscured by clouds. When the class arrived, the mountains were clear and we could easily observe the glaciers that scour the upper reaches of the mountains. Glaciers technically shouldn't exist here. Although we were at a high enough latitude, the nearby Pacific Ocean moderates the climate, keeping things warmer than they would otherwise be (the Olympics are at the same latitude as Great Falls, Montana, or St. Paul, Minnesota). But temperature isn't the only factor in glacier development. The sheer amount of snowfall in combination with temperatures that are just cold enough allows glaciers to exist at these unusually low elevations.
We headed down to Port Angeles for lunch and to find to road to the other major locale the day: Neah Bay and lands of the Makah people.